The invention relates to a catalyst for fuel cells, in particular to a carbon-supported anode catalyst based on platinum/ruthenium for direct methanol fuel cells (DMFC).
Fuel cells convert a fuel and an oxidant in separate locations at two electrodes into electric power, heat and water. The fuel used can be hydrogen, methanol or a hydrogen-rich gas, and the oxidant can be oxygen or air. The energy conversion process in the fuel cell is largely free of pollutants and has a particularly high efficiency. For this reason, fuel cells are becoming increasingly important for alternative propulsion concepts, domestic energy supply plants and portable applications. The direct methanol fuel cell (DMFC) converts the fuel methanol directly into electric energy. Owing to its low operating temperature, its compact construction and its power density, it is particularly suitable for portable applications, e.g. as replacement for accumulators and batteries.
PEM fuel cells and DMFCs are made up of a stack of many fuel cell units. These are electrically connected in series to increase the operating voltage. With regard to the structure and mode of operation of fuel cells, reference may be made to the relevant literature (e.g. K. Kordesch, G. Simader, “Fuel Cells and its Applications”, VCH-Verlag Chemie, Weinheim 1996).
The key component of a PEM fuel cell or DMFC is the Membrane-Electrode-Unit (MEU). The MEU generally comprises five layers, viz. the proton-conducting membrane (polymer electrolyte membrane or ionomer membrane), the two gas diffusion layers (GDLs or “backings”) on the sides of the membrane and the two electrode layers located between membrane and gas diffusion layers. One of the electrode layers is configured as anode for the oxidation of methanol and the second electrode layer is configured as cathode for the reduction of oxygen. The gas diffusion layers (GDLs) usually comprise carbon fibre paper or woven carbon fibre fabric and make it possible for the reactants to get to the reaction layers readily and allow the cell current and the water formed to be conducted away readily.
In the DMFC, methanol (or an aqueous methanol solution) is converted directly into CO2, water and electric current. This is referred to as a “liquid feed” arrangement. The corresponding reactions are:Anode: CH3OH+H2O→CO2+6H++6 e−Cathode: 3/2O2+6H++6 e−→3H2OOverall reaction: CH3OH+3/2 O2→CO2+2H2O
The electrode layers for the anode and cathode of the DMFC comprise a proton-conducting polymer and electrocatalysts which catalyse the respective reaction (oxidation of methanol or reduction of oxygen). A bimetallic platinum/ruthenium catalyst is preferably used as catalytically active component on the anode, and a platinum catalyst is preferably used on the cathode side. Use is made mainly of supported catalysts in which the catalytically active precious metals have been applied in finely divided form to the surface of a conductive, carbon-based support material, for example carbon black or graphite. However, it is also possible to use Pt and/or PtRu powders (known as precious metal blacks).
The peak power density achieved at present in DMFCs is still too low for practical applications. The great challenges in the development of the technology are therefore improvement of the power density, prevention of the methanol transport through the membrane to the cathode side (“MeOH crossover”) and reduction of the usage of catalyst containing precious metals, in particular PtRu catalyst on the anode side.
The present invention is concerned with high-loading, supported platinum/ruthenium (PtRu/C) catalysts for use as anode catalysts for direct methanol fuel cells (DMFCs). These high-loading supported catalysts are used in the form of catalyst inks for producing electrodes and membrane-electrode units and lead to a significant improvement in performance of the DMFC.
Electrocatalysts supported on carbon black and having up to 90 wt. % of PtRu on the support material have been described by K. A. Friedrich et al. in “Journal of Electroanal. Chemistry”, Volume 524-525 (2002), pages 261-272. They are prepared by the “sulfito method” and the particle sizes of these catalysts prepared in this way are about 6 nm (measured by means of XRD).
EP 952 241 B1 describes PtRu catalysts having a PtRu content of from 10 to 80 wt. % on the carbon black support material, while the patents EP 880 188 B1 and EP 924 784 B1 disclose PtRu electrocatalysts having PtRu contents of from 10 to 40 wt. % or from 10 to 50 wt. % (in each case based on the total weight of the catalyst).
Furthermore, EP 1 260 269 A1 discloses a process for preparing PtRu catalysts which have a precious metal content of from 10 to 80 wt. % and are used in PEM fuel cells.
A person skilled in the art will know that high precious metal contents (i.e. contents above 60 wt. % based on the total weight of the catalyst) lead to coarsening of the catalyst particles and thus to a lower catalytically active catalyst surface area. Furthermore, a poor, non-uniform distribution of the precious metal particles on the carbon black support is obtained, resulting in a drop in the utilization of the catalyst. All these factors finally have an adverse effect on the electric power of the fuel cell. Supported catalysts which have been prepared by conventional methods generally have relatively coarse particles (i.e. particles having a mean particle size measured by XRD of significantly above 3 nm) at precious metal contents of above 60 wt. % of PtRu and therefore suffer from the disadvantages indicated.